Isolated my mp3 in audio noise a bit further. The audio is fine using my phone headphone out. The problem is when I use my laptop headphone out jack. The noise is terrible. I hear no issue when I have headphones plugged into the laptop. I have Band-in-a-Box that I want to use for practicing, but until I can find a way to clean up the signal the best I've been able to do is connect earbuds to the Stagg and then wear can headphones over those connected to the laptop. Irksome.
Update:
Found an old USB C-Media audio dongle that I had laying around. It seems to work just fine, so I'm running through that instead of the laptop's built-in audio. No more double layered headphones.